Numerous sensor elements and methods for detecting at least one property of a measuring gas in a measuring gas chamber are known from the related art. This may basically involve any physical and/or chemical properties of the measuring gas, and one or multiple properties may be detected. The present invention is described below in particular with regard to a qualitative and/or quantitative detection of a proportion of a gas component in the measuring gas, in particular with regard to a detection of an oxygen content in the measuring gas. The oxygen content may be detected in the form of a partial pressure and/or in the form of a percentage, for example. Alternatively or additionally, however, other properties of the measuring gas, such as the temperature, are also detectable.
For example, sensor elements of this type may be designed as so-called lambda sensors, as are known, for example, from Konrad Reif (Ed.): Sensoren im Kraftfahrzeug [Automotive Sensors], 1st Edition 2010, pp. 160-165. Using broadband lambda sensors, in particular planar broadband lambda sensors, the oxygen concentration, for example, in the exhaust gas may be determined over a wide range, and the air-fuel ratio in the combustion chamber may thus be deduced. The excess air factor λ describes this air-fuel ratio.
In particular ceramic sensor elements are known from the related art which are based on the use of electrolytic properties of certain solid bodies, i.e., based on ion-conducting properties of these solid bodies. In particular, these solid bodies may be ceramic solid electrolytes, for example zirconium dioxide (ZrO2), in particular yttrium-stabilized zirconium dioxide (YSZ) and scandium-doped zirconium dioxide (ScSZ), which may contain small additions of aluminum oxide (Al2O3) and/or silicon oxide (SiO2).
Increasing functional requirements are being imposed on such sensor elements. In particular, rapid operational readiness of lambda sensors after starting the engine plays a large role. This is influenced essentially by two aspects. The first aspect concerns rapid heating up of the lambda sensor to its operating temperature, which is customarily above 600° C., which may be achieved by an appropriate design of a heating element or a reduction in size of the area to be heated. The other aspect concerns the robustness against thermal shock due to hydrolock during operation. This thermal shock is based on the fact that the temperature in the exhaust pipe is below the dew point of water for a certain period after starting the engine, so that the water vapor which forms during the combustion of fuel may condense in the exhaust pipe. This results in the formation of water droplets in the exhaust pipe. The heated-up ceramic of the lambda sensor may be damaged or even destroyed due to thermal stresses or ruptures in the sensor ceramic due to the impact of water droplets.
For this reason, lambda sensors have been developed which have a porous protective ceramic layer on their surface, also referred to as a thermal shock protective layer. This protective layer ensures that water droplets which strike the lambda sensor are distributed over a large surface area, and therefore the localized temperature gradients occurring in the solid-state electrolyte or the sensor ceramic are reduced. In the heated state, these lambda sensors thus tolerate a certain size of condensation water droplets without being damaged. The protective layer is customarily applied to the sensor element in an additional method step. Various materials, for example aluminum oxide or spinel (MgAl2O4), and application techniques, for example spraying or dipping processes, are used for this purpose. For example, it is known to apply a uniformly thick thermal shock protective layer made of porous aluminum oxide with the aid of atmospheric plasma spraying. With this type of thermal coating process, introduced particles are melted and are accelerated onto the solid electrolyte surface, so that the thermal shock protective layer is applied to the entire solid electrolyte surface. In the low-temperature range, i.e., in a temperature range of approximately 300° C. to 400° C., due to its limited permeability the thermal shock protective layer reduces the entry of water into the solid electrolyte of the sensor element, which is made at least partially of zirconium dioxide, and in the high-temperature range, i.e., in a temperature range of approximately 400° C. to 600° C., limits the cooling via heat conduction. At higher temperatures, the Leidenfrost effect prevents cooling.
Despite the numerous advantages of the methods known from the related art for manufacturing sensor elements for lambda sensors, these methods still have the potential for improvement. In order to not influence the functionality of the sensor element and at the same time to reliably protect against water droplets, for example from the exhaust gas flow of an internal combustion engine, the thickness and the porosity of the thermal shock protective layer must be optimally selected. Optimizing the sensor element with regard to the two mentioned influencing variables results in various conflicting objectives. Thus, a thick thermal shock protective layer reliably protects against hydrolock, but as additional mass, adversely affects the heat-up behavior of the sensor element. The thermal shock protective layer impairs the dynamics of the lambda sensor. In addition, the use of aluminum oxide as a thermal shock protective layer material having good heat conductivity may result in increased heat discharge from the sensor element. Lastly, although slimming down the ceramic substrate allows quicker heat-up times, it makes the sensor element more mechanically fragile. In addition, the sprayed layers are relatively inhomogeneous, so that the layers must be sprayed on more thickly than necessary in order to be sufficiently stable against thermal shock. Moreover, the open porosity of plasma-sprayed layers changes due to thermal aging, so that the functioning of “voltage-jump” sensors as well as broadband sensors having a covered gas inlet port is impaired. The connection between the sensor element and the layer provided by the inherent bond of the thermal shock protective layer is not entirely satisfactory. In addition, the thermal shock protective layer represents an additional diffusion barrier through which the measuring gas, which contains water vapor and carbon dioxide, for example, must diffuse in order to reach the outer electrode.